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A. Zhang and C. Li
a diverse prokaryotic lineage whose ancestor gave rise to the plant chloroplast [25].
More than 40,000 species of microalgae have been described, and they collectively
cover a comprehensive spectrum of habitats and tolerances of ranges of pH, salinity,
and temperature [26–29] estimated that prokaryotic and eukaryotic microalgae are
responsible for more than 40% of net primary productivity on Earth [26–29]. Algae
can be a more appealing biofuel feedstock than land plants because of their faster
biomass doubling cycle, their more accessible forms of stored carbon than the lignocelluloses used for cellulosic biofuels, and their ability to thrive on water sources
and on land sites that are unsuitable for terrestrial farming.
Microalgae contain diverse pigments and metabolites that are desirable as nutritional supplements and colorants. Examples of such products include astaxanthin,
an antioxidant derived from the alga Haematococcus, and a high-protein powder
derived from cyanobacterial species of Spirulina (Arthrospira) [30, 31]. Commercial scale algal ponds that grow these and other microalgae have operated for more
than a decade [32]. However, the scale of deployment for algae cultivation for fuel
is expected to be much larger than the scale of algae cultivation for nutraceuticals or
other specialty products currently available in the market.
Generating biofuels from algae requires exploiting and expanding the demonstrated commercial scale growth of algal biomass, and harvesting the relatively
accessible carbon stored therein. Carbon is stored within algal cells in various forms,
and these molecules can be accessed by different technologies. Both eukaryotic
and prokaryotic algal cells are rich sources of polar lipids that are associated with
membranes; in some cases, the photosynthetic thylakoid membranes are extensive.
Carbon is such a crucial element for algae that it is typical for them to store surplus
carbon when cellular division is restricted by some factor other than carbon availability—this situation is termed unbalanced growth. In many eukaryotic microalgae, photosynthetic carbon fixation continues under unbalanced conditions. Under
extended periods of environmental stress, the excess fixed carbon is stored in the
form of neutral lipids called triacylglycerols (TAGs). TAGs are hydrocarbon chains
terminated in a carboxylic acid group. The three carboxyl groups are bound to glycerol through an ester linkage. Biofuels containing hydrocarbon chains longer than six
carbons are particularly valued because of their high heats of combustion, volatility,
and compatibility with existing engines. As discussed later in this chapter, extracted
TAGs can be converted to biodiesel using a number of technologies, including transesterification and hydrotreating. Even algal species that do not store large amounts
of TAGs can be converted to biofuels through various chemical conversion technologies. For example, species that store polysaccharides can be fermented to yield
ethanol, and other biomass processing technologies, such as gasification, pyrolysis,
and hydrothermal liquefaction, have shown great utility for the conversion of whole
biomass into biofuels.
The incipient algal biofuel industry is emerging and evolving from its early foundations in algae cultivation for fish feedstuff and for human nutraceuticals. Early
technology development of processing algae to fuels emphasized the conversion of
neutral lipids (TAGs) to biodiesel. Choices of algal feedstock have been expanding to
address the goals of fuel production rather than nutritional content and to exploit new
A. Zhang and C. Li
a diverse prokaryotic lineage whose ancestor gave rise to the plant chloroplast [25].
More than 40,000 species of microalgae have been described, and they collectively
cover a comprehensive spectrum of habitats and tolerances of ranges of pH, salinity,
and temperature [26–29] estimated that prokaryotic and eukaryotic microalgae are
responsible for more than 40% of net primary productivity on Earth [26–29]. Algae
can be a more appealing biofuel feedstock than land plants because of their faster
biomass doubling cycle, their more accessible forms of stored carbon than the lignocelluloses used for cellulosic biofuels, and their ability to thrive on water sources
and on land sites that are unsuitable for terrestrial farming.
Microalgae contain diverse pigments and metabolites that are desirable as nutritional supplements and colorants. Examples of such products include astaxanthin,
an antioxidant derived from the alga Haematococcus, and a high-protein powder
derived from cyanobacterial species of Spirulina (Arthrospira) [30, 31]. Commercial scale algal ponds that grow these and other microalgae have operated for more
than a decade [32]. However, the scale of deployment for algae cultivation for fuel
is expected to be much larger than the scale of algae cultivation for nutraceuticals or
other specialty products currently available in the market.
Generating biofuels from algae requires exploiting and expanding the demonstrated commercial scale growth of algal biomass, and harvesting the relatively
accessible carbon stored therein. Carbon is stored within algal cells in various forms,
and these molecules can be accessed by different technologies. Both eukaryotic
and prokaryotic algal cells are rich sources of polar lipids that are associated with
membranes; in some cases, the photosynthetic thylakoid membranes are extensive.
Carbon is such a crucial element for algae that it is typical for them to store surplus
carbon when cellular division is restricted by some factor other than carbon availability—this situation is termed unbalanced growth. In many eukaryotic microalgae, photosynthetic carbon fixation continues under unbalanced conditions. Under
extended periods of environmental stress, the excess fixed carbon is stored in the
form of neutral lipids called triacylglycerols (TAGs). TAGs are hydrocarbon chains
terminated in a carboxylic acid group. The three carboxyl groups are bound to glycerol through an ester linkage. Biofuels containing hydrocarbon chains longer than six
carbons are particularly valued because of their high heats of combustion, volatility,
and compatibility with existing engines. As discussed later in this chapter, extracted
TAGs can be converted to biodiesel using a number of technologies, including transesterification and hydrotreating. Even algal species that do not store large amounts
of TAGs can be converted to biofuels through various chemical conversion technologies. For example, species that store polysaccharides can be fermented to yield
ethanol, and other biomass processing technologies, such as gasification, pyrolysis,
and hydrothermal liquefaction, have shown great utility for the conversion of whole
biomass into biofuels.
The incipient algal biofuel industry is emerging and evolving from its early foundations in algae cultivation for fish feedstuff and for human nutraceuticals. Early
technology development of processing algae to fuels emphasized the conversion of
neutral lipids (TAGs) to biodiesel. Choices of algal feedstock have been expanding to
address the goals of fuel production rather than nutritional content and to exploit new